1. Field of the Invention
The present invention relates to a semiconductor imaging device employing lateral MOS static induction transistors.
2. Description of the Prior Art
Charge transfer devices such as BBDs and CCDs, MOS transistors and the like have heretofore been widely used as a semiconductor imaging device of the type constituted by a semiconductor photoelectric conversion device, the semiconductor imaging device being typically incorporated in an electronic camera, a home video camera, a facsimile system or the like. However, a typical semiconductor imaging device using such devices involves various problems in that the leakage of electric charge may take place while a signal electric charge is being transferred, in that its photoelectric sensitivity is inferior, and in that the degree of circuit integration is low.
In order to solve the problems, proposals have heretofore been made with respect to use of a semiconductor imaging device of the type employing static induction transistors (hereinafter referred to simply as "SIT" or "SITs"). The SIT is a kind of phototransistor which has the function of photoelectric conversion and the function of amplifying photoelectric charge. As compared with field effect transistors and junction transistors, the SIT possesses various advantages such as high input impedance, high-speed operation, non-saturability, low noise and low power consumption.
Accordingly, if the SIT is utilized as a photosensitive element, it is possible to obtain a semiconductor imaging device having high photoelectric sensitivity, high-speed response and a wide dynamic range.
It is known that so-called vertical junction SITs, lateral MOS SITs or the like are used in such a semiconductor imaging device employing SITs. A semiconductor imaging device employing vertical junction SITs is disclosed in the specification of Japanese Pat. Laid-open No. 15229/1980.
FIG. 1 diagrammatically shows the sectional structure of one of the SITs which respectively constitute the picture elements of the known semiconductor imaging device. As shown, the SIT has a vertical structure in which an n.sup.+ -type substrate 1 serves as a drain region, an n.sup.- -type epitaxial layer 2 which constitutes a channel region being deposited upon the substrate 1 and a source region 3 constituted by an n.sup.+ -type region being formed in the surface of the epitaxial layer 2. Moreover, a signal storage gate region 4 of a p.sup.+ type is formed in the surface of the epitaxial layer 2 such that the region 4 surrounds the source region 3. An electrode 6 is formed on the gate region 4 with an insulating film 5 being interposed therebetween, and this constitutes a so-called MIS (metal insulator semiconductor) structure which typically includes an electrode, an insulating film and a gate region. Incidentally, isolating regions 7 are so formed as to separate the respective SITs which constitute picture elements.
However, in each of the SITs having the above-described vertical structure, it is necessary to increase the thicknesses of the epitaxial layer 2 and the gate region 4 in order to enhance an amplification factor with respect to incident light. In addition, it is also necessary to separate the respective signal electric charges produced in the SITs by disposing the isolating regions 7 between the respective SITs of the semiconductor imaging device which is constructed in the aforesaid manner. In order to enable this separation, it is a common practice to use several known methods such as separation based on the use of oxide film, separation obtained by diffusion, separation based on the use of V-shaped trenches and the like. In this case, the isolating regions 7 are formed such as to extend from the surface of the epitaxial layer 2 to the substrate 1. Therefore, if the thickness of the epitaxial layer 2 is increased in order to enhance the amplification factor with respect to incident light, this makes it more difficult to form the isolating regions 7. Diffusion or similar method have limitations when the gate region 4 is to be deeply formed for the purpose of enhancing the amplification factor. If the gate region 4 is deepened, photoabsorption may take place in the gate region 4 and this could result in a lowering of photosensitivity. For these reasons, the semiconductor imaging device including the SITs having the vertical structure involves a limitation when the amplification factor or the photosensitivity is to be improved. This disadvantage cannot possibly be avoided in terms of the structure of such a device in itself.
In order to solve the aforesaid disadvantage, the present applicant previously disclosed a semiconductor imaging device including lateral MOSSITs in the specifications of Japanese Pat. Laid-open Nos. 140752/1985 and 206063/1985 and a treatise entitled "A New MOS Phototransistor Operating in a Non-Destructive Readout Mode", published in Japanese Journal of Applied Physics, Vol. No. 5, 1985.
The following is a description of the photosensitive elements of a semiconductor imaging device including the lateral MOSSITs each having a prior-art MIS gate structure.
FIGS. 2A and 2B respectively are a top plan view and sectional view of a lateral MOSSIT type photosensitive element which constitute each of the picture elements of a known semiconductor imaging device proposed by the present applicant. The structure, parameters and the like of the lateral MOSSIT photosensitive element of an n-channel type will be described below. In FIGS. 2A and 2B, a p.sup.- type ,100. substrate is indicated at 11, and the substrate 11 has a concentration of about 10.sup.12 to 10.sup.15 cm.sup.-3. An n.sup.- type epitaxial layer 12 is grown on the substrate 11 by an epitaxial method, and constitutes n.sup.31 type channel regions. The aforesaid n.sup.- -type epitaxial layer 12 has a thickness of about 5 to 10 .mu.m and a concentration of about 1.times.10.sup.13 to 5.times.10 .sup.13 cm.sup.-3. The n.sup.- type epitaxial layer 12 has a surface including an n.sup.+ -type drain region 13 and an n.sup.+ -type source region 15, the regions 13 and 15 being respectively made of shallow diffused layers. In this case, the n.sup.+ -type drain region 13 and the n.sup.+ -type source region 15 are formed in a self-aligned manner with respect to a gate electrode 17 which will be described later.
The depth of diffusion in the n.sup.+ -type drain region 13 is equal to or less than about 0.5 .mu.m. A gate insulating film 16 is formed on the n.sup.- -type epitaxial layer 12, and has a thickness of about 200 to 1000 .degree. .ANG.. The gate electrode 17 is formed on the gate insulating film 16, and the aforesaid n.sup.+ -type drain region 13 and n.sup.+ -type source region 15 are formed in a self-aligned manner with respect to the gate electrode 17. The gate electrode 17 is formed by, for example, a polysilicon film having a thickness of about 5000 .ANG. or less. A drain electrode 18 is formed on the n.sup.+ -type drain region 13 and a source electrode 19 is formed on the n.sup.+ -type source region 15. An insulating film is indicated at 20, and an isolating region 21 is formed so as to separate the respective picture elements, the region 21 made of an insulating film being formed by a trench forming method.
The operation of the aforesaid lateral MOSSIT type photosensitive element will be described in brief with reference to FIGS. 2A and 2B. When a minus voltage (not shown) is applied to the gate electrode 17, a signal electric charge is generated in response to incident light 22 which is incident from above the gate electrode 17. The signal electric charge is stored in the surface of the n.sup.- -type channel region within the n.sup.- -type epitaxial layer 12, which surface is defined directly below the gate electrode 17. The storage of the signal electric charge enables modulation of electron current flowing between the n.sup.+ -type source region 15 and the n.sup.+ -type drain region 13 in the n.sup.- -type channel region.
However, in a semiconductor imaging device of the type utilizing the above-described lateral MOSSITs, the source and drain regions are formed for each of the picture elements, and, in addition, each group of the source, gate and drain regions is surrounded by the isolating region. This makes it difficult to reduce the size of each of the picture elements.